I know that perpetual motion machines can't work because of friction, but intuitively I thought that Escher's waterfall (https://cdn.churchm.ag/wp-content/uploads/2011/04/waterfall-illusion-e1302170121549.jpg) couldn't work because of the absence of external energy. I can't see where friction prevents this process from working. Any ideas? Thanks in advance and have a good day :)
closed as unclear what you're asking by sammy gerbil, stafusa, Cosmas Zachos, Chris♦, valerio Feb 19 '18 at 9:01
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Escher's waterfall doesn't work because you can't fall off a cliff, then go downhill and end up where you started. It's geometry, not physics.